Abstract

According to the traditional point of view any collective phenomena can exist either in systems of particles with a long-range interaction between them (e.g. the Langmuir oscillations in the Coulomb plasma etc.) or in the case of high enough density systems in which the mean interparticle distance is of the order of the interaction range (spin waves in condensed magnets etc.). If some low density system is considered the collective effects may appear in the system only at low enough temperatures compared with the quantum degeneracy temperature εd.